This application incorporates by reference of Taiwan application Serial No. 90131457, Filed Dec. 19, 2001.
1. Field of the Invention
The invention relates in general to a planar antenna, and more particularly to a planar inverted-F antenna.
2. Description of the Related Art
As the technology progresses, it makes people""s daily life much easier. In terms of the communication technology, it leads to communication between people almost without the limitation of distance and time. In the past, wired domestic telephones and public telephones were commonly used for communication. They are convenient to use, but they have the disadvantage of lacking mobility. Thus, real-time communicating with people would be impossible in some situations. For this reason, pagers are developed to supplement the requirements of mobile communication. Recently, mobile phones are used more frequently than the pagers. Users can immediately make and receive a call by mobile phones. Further, users can even connect to the Internet for browsing information, sending and receiving electronic mails through the use of wireless application protocol (WAP). With these versatile functions, mobile phones are consequently standard personal communication equipments. The key to the popularity of mobile phones depends on their compact sizes, innovative functions, and affordable costs. Strictly speaking, the technology of circuit manufacturing determines all of these conditions. If the technology of circuit manufacturing is mature, the mobile phones can be more compact. In addition, the compact mobile phones contribute to their popularity, resulting in mass production and hence lowering the production cost. In this way, how to develop more compact circuitry is an important subject for engineers and researchers in this industry.
As discussed above, in terms of the integrated circuit development, the current and future trend is towards miniaturization. Thus, wireless communication products are invariably towards this trend. Antennas, the key components of the circuitry of wireless communication products, have to be minimized. When the antenna is in resonance at a resonance frequency, there will be an EM wave excited corresponding to the resonance frequency. The operating length of the antenna is decided by the wavelength (xcex) of the resonance frequency. The operating length of the conventional antenna used in the wireless communication products, such as the dipole antenna or the microstrip patch antenna, is one-half of the wavelength (xcex/2) of the resonance frequency. In recent years, the planar inverted-F antenna (PIFA) structure has been developed. The operating length can be decreased to one-fourth of the wavelength (xcex/4) of the resonance frequency when using the planar inverted-F antenna in the wireless communication products. Therefore, the size of the antenna can be decreased. Besides, the planar inverted-F antenna can be placed above the ground plane and embedded within the housing of the mobile phone. Therefore, the purpose of hiding the antenna for the mobile phone can be achieved.
Referring now to FIG. 1, it illustrates the structure of the planar inverted-F antenna 100. The planar inverted-F antenna includes a radiating device 110, a ground surface 130, a dielectric material 150, a shorting device 170, and a feeding device 190. The dielectric material 150 is set between the radiating device 110 and the ground surface 130 for isolating the radiating device 110 from the ground surface 130. In practice, the dielectric material 150 can be air, a polystyrene, a substrate, or the combination of the above-disclosed materials. The radiating device 110 is coupled to the ground surface 130 through the shorting device 170. The shorting device 170 can be a simple metallic pin or other devices. The feeding device 190 can be set on the ground surface 130 and coupled to radiating device 110 for transmitting microwave signals. The feeding device 190 can be a SMA connector or other devices. The radiating device 110 and the ground surface 130 can be made of metallic materials. The operating length of the planar inverted-F antenna can be as short as one-fourth of the wavelength (xcex/4) of the resonance frequency. Therefore, when using the planar inverted-F antenna in the wireless communication products, the size of the antenna can be decreased.
The system of the common dual-frequency mobile phone is GSM 900 or GSM 1800 system. In other words, the resonance frequency of the antenna in most mobile phones is 900 MHz or 1800 MHz. Since the size of the mobile phone is getting smaller and smaller, the size of the antenna must be decreased without affecting the performance of the antenna. Basically, the structure of each inverted-F antenna is substantially the same, as shown in FIG. 1. The difference of each inverted-F antenna is the pattern of the radiating device. The resonance frequency of the planar inverted-F antenna is decided by the pattern of the radiating device. Therefore, the design of the pattern of the radiating device is very important.
Referring now to FIG. 2A, it illustrates the conventional pattern design of the radiating device of the dual-frequency planar inverted-F antenna. The radiating device 210A is coupled to the shorting device at the ground connecting point 271. The radiating device 210A is coupled to the feeding device at the feeding point 291. For simplicity, the ground connecting point 271 is represented by a square and the feeding point 291 is represented by a triangle in FIG. 2 and the following figures. In FIG. 2A, the radiating device 210A includes an L-shape slot. It is obvious that, when the radiating device 210A is excited, there are two different effective surface current paths (L1 and L2) on the radiating device 210A. As shown in FIG. 2A, the length of the effective surface current path (L1) is different from that of the effective surface current path (L2). The shorter current path (L1) makes the antenna have a higher resonance frequency such as 1800 MHz, and the longer current path (L2) makes the antenna have a lower resonance frequency such as 900 MHz. In this manner, the antenna can be operated at both 900 MHz and 1800 MHz, and can be used in the dual-frequency mobile phone. Referring now to FIG. 2B, it illustrates another conventional pattern design of the radiating device of the dual-frequency planar inverted-F antenna. The radiating device 210B includes a U-shaped slot. When the radiating device 210B is activated, there are also two different effective surface current paths (L1 and L2), and the length of the effective surface current path (L1) is different from that of the effective surface current path (L2). The shorter effective surface current path (L1) makes the antenna have a higher resonance frequency, and the longer effective surface current path (L2) makes the antenna have a lower resonance frequency.
Referring now to FIG. 3A, it illustrates the diagram of the return loss of the conventional planar inverted-F antenna. The operating bandwidth of the antenna is defined to be 2.5:1 of the voltage standing wave ratio (VSWR). If the resonance frequency of the antenna is f0, the ideal bandwidth of the antenna is BW1, as shown by the real line in FIG. 3A. However, in order to decrease the size of the antenna, the radiating device is set near the ground surface in the practical design. In this manner, the bandwidth of the antenna is narrowed. The practical bandwidth of the antenna is shown by the dashed line in FIG. 3A. The practical bandwidth is narrower than the ideal bandwidth. To sum up, the purpose of decreasing the size of the antenna can be achieved through lowering the position of the radiating device. However, the bandwidth of the antenna is narrowed, when lowering the position of the radiating device to the ground surface.
When the L-shape (shown in FIG. 2A) or the U-shape slot (shown in FIG. 2B) is set on the radiating device, the planar inverted-F antenna can have two different resonance frequencies. (xe2x80x9cNew slot configuration for dual-band planar inverted-F antennaxe2x80x9d, Microwave Optical Technology Letters, vol. 28, no. 5, Mar. 5, 2001, pp. 293-298). However, the operating bandwidth of the antenna is usually too narrow to satisfy the bandwidth requirement of the GSM 900 and GSM 1800 systems at the same time if the radiating device includes an L-shape or U-shape slot. There are also some other conventional methods for designing the pattern of the radiating device of the planar inverted-F antenna such as feeding capacitively as disclosed in U.S. Pat. No. 5,764,190. However, the structure of the antenna is too complicated and the cost of manufacturing is too high when using the above-disclosed method to design the antenna structure.
It is therefore an object of the invention to provide an improved and simplified planar inverted-F antenna with the following advantages. First, the operating bandwidth of the antenna is broad. Second, the size of the antenna can be decreased. Third, the structure of the antenna can be simplified.
The invention achieves the above-identified objects by providing a planar inverted-F antenna (PIFA) with a first operating bandwidth and a second operating bandwidth. The planar inverted-F antenna includes a ground surface, a radiating device, a shorting device, a dielectric material, and a feeding device. The dielectric material is set between the radiating device and the ground surface for isolating the radiating device from the ground surface. The feeding device is set on the ground surface and coupled to the radiating device for transmitting a microwave signal. The radiating device is coupled to the ground surface through the shorting device. The radiating device further includes a first radiating element, a second radiating element, and a third radiating element. The first radiating element has a first resonance mode and a second resonance mode. The second radiating element has a first resonance mode and a second resonance mode. The third radiating element has a first resonance mode. The first operating bandwidth of the planar inverted-F antenna is formed by the first resonance mode of the first radiating element and the first resonance mode of the second radiating element. The second operating bandwidth of the planar inverted-F antenna is formed by the second resonance mode of the first radiating element, the second resonance mode of the second radiating element, and the first resonance mode of the third radiating element.